Knoop Hardness

Many times it is required to determine the hardness over a very small region or a very small part. The indenters of methods described previously may be larger than the area of part over which hardness is to be determined. The individual constituents of a microstructure, delicate gear of a watch or the area where sharp hardness gradient exists may be such examples. The solution to such a problem may be using a very small indenter under a small load.

                               Figure: Knoop Indenter and Indentation

Knoop hardness measurement is such a method in which a diamond indenter having pyramidal faces with very large included angle is used. This indenter is pressed against a metallographically finished surface to produce an indentation of the diamond shape . The longer diagonal of the impression is approximately seven times the shorter diagonal. The depth of indentation is about one thirteenth of the longer diagonal. The Knoop Hardness Number (KHN) is the measure of the ratio of load to projected area of impression.
Thus,

                                               KHN =    P / d ² C                                           ----------- (21)

where d is the length of longer diagonal and C is a factor of diagonal pyramid, supplied by the manufacturer. The indentation load is very small, often as low as 30 gms. The small impression is measured under microscope and also since the area over which the hardness measurement is made is very small, KHN is sometimes called microhardness.

It is highly essential that the surface of sample must have metallographic finish as well as the indenting load controlled carefully. Any error in measurement of d can cause error in hardness. The error in locating ends of the impression becomes larger at lower load and hence the hardness has a tendency to increase with reduction in load and corrections for such error are available.